Water is a Chemical!?

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Warm-Up: What is weird about water?
Watch video clip and complete the first
column in the 3-column worksheet
Cloze Reading

Read Aloud:

Textbook: Molecules at an Exhibition page 169170. By Emsley J. (1998)

Complete the second and third column of
the 3-column worksheet as you listen to
the article.
Criss Cross Puzzle

Use the textbook read to complete this
practice.
Criss Cross Puzzle feedback
1. Sonochemistry
 2. Boils
 3. honeycomb
 4 Oxygen
 5. Down
 6. Pipes
 7. Supercritical
 8. Freezes
 9. Hydrogen

10.
11.
12.
13.
Framework
Cavendish
Microbes
Expand
Video Analysis

Re-watch the video clip and complete the
structured video analysis worksheet #1-4
Interactive Mini-Lecture
Take out a new note sheet and start
taking note now
Water is a Chemical!?
H2O

Indeed! Water is one of our most plentiful
chemicals.

Its chemical formula, H2O, is probably the
most well known of all chemical formulas.
What does the chemical formula
tell us?

The formula H2O tells us that one
molecule of water is comprised of 2
atoms of hydrogen and one atom of
oxygen bonded together.

The bonds which hold the hydrogen
and oxygen together are called
covalent bonds - they are very
strong.
Let's look at a picture of a molecule of water: In
this picture the two hydrogen atoms are
represented by white spheres and the oxygen by
a red sphere.
Why does the water molecule
look bent?

The water molecule maintains a bent
shape because of two considerations.

First the tetrahedral arrangement
around the oxygen and

Second the presence of lone pair
electrons on the oxygen.
Did I say "Tetrahedral" - What
does that mean?
• Tetrahedral means "four-sided".
• This molecular shape is shown below. It
has regular bond angles of 109.5 degrees

However, there is one important
difference - the bond angles for water
are not 109.5. Because of the presence
of the very negative lone pair electrons,
the two hydrogen atoms are squeezed
together as the two lone pairs try to get
away from each other as far as possible.

The resulting angle gives water a 104.5
bond angle resulting to tetrahedron
"looks" BENT!
What are Lone Pair Electrons?

These are the electrons that are not
involved in the covalent bonds.The pairs
of electrons are left alone. In our picture
they are represented by the double dots.

-

These lone pairs are very negativecontaining two negative electrons each - and
want to stay away from each other as much
as possible.

These repulsive forces act to push the
hydrogen atoms closer together
The lone pairs are negative - Are
the Hydrogens positive?

The hydrogens are slightly positive. They get this
way because of the "electronegativity" of oxygen.

Electronegativity is a measure of how much one
atom wants to have electrons, and oxygen wants
to have electrons more than hydrogen does.

Oxygen has a higher electronegativity. Because of
this difference in electronegativity, the electrons
in the covalent bonds between oxygen and
hydrogen get pulled slightly toward the oxygen.
This leaves the hydrogen atoms a little bit
electron-deficient and thus slightly positive.
 We can draw this polarization like this:

Or looking at it from a "net
polarization" perspective, like this:
What does the polarization have to
do with the properties of water?

Everything! Because water has a slightly negative
end and a slightly positive end, it can interact with
itself and form a highly organized 'inter-molecular'
network.

The positive hydrogen end of one molecule can
interact favorably with the negative lone pair of
another water molecule.

This interaction is call "Hydrogen Bonding". It is a
type of weak electrostatic attraction (positive to
negative). Because each and every one of the
water molecules can form four Hydrogen Bonds,
an elaborate network of molecules is formed.
Ball-and-Stick Model of a water
molecule

And to interact with other polar molecules which is how substances become dissolved in
water.
Does this make water unusual?

YES! But it's not just that the molecule is
bent that makes it special. Water is also
highly polar - the two sides of water have
very different charge.
Why Ice blocks float on water (Titanic)
 Why insects can walk on water
 Why water is called a “universal solvent”

But if the Hydrogen Bonds are
weak, how can they be important?
Think of how many there are! There is
strength in numbers!
 The polarity also allows water interact
with an electric field:

References
IMAGE SOURCE: "Chemistry in Context"
Wm C Brown Publishers, Dubuque Iowa,
2nd edition, A project of the American
Chemical Society, ed: A.Truman Schwartz
et al., 1997, Chapter 5 "The Wonder of
Water"
 IMAGE SOURCE: "Chemistry and Life",
4th Edition, John W. Hill, Dorothy M. Feigl,
and Stuart J. Baum, Macmillan Publishing
Company, New York, 1993
